Add `cht-conf` support for `delete-ui-extensions` action

[jkuester]

Add a new action: src/fn/delete-ui-extensions.js. Should be a pretty easy operation of just removing the document for the specified extension. See the delete-forms action as an example.

[jkuester]

TODO What happens to the service worker when this is deleted?

[Vedd-Patel]

@jkuester @andrablaj
i would love to work on this!
can you pls assign me this?